The invention relates to a novel method of preparing anhydrous crystalline maltitol.
Maltitol (xcex1(1xe2x86x924)glucosylsorbitol) is a sugar alcohol generally used as a sweetening agent in low-caloric, dietary and low-cariogenic foods, such as confectionary products and chewing gums. Maltitol has a sweet taste similar to that of sugar, and the sweetening power of crystalline maltitol with high purity has a sweetness of about 85 to 95% of that of sucrose, making it sweeter than all other polyols except xylitol.
Maltitol is produced from a starch solution, which is first enzymatically hydrolyzed into maltose syrup. After purification and concentration, the maltose syrup is catalytically hydrogenated to maltitol. Following additional purification steps to remove the starting materials and the catalyst, the solution is concentrated to a syrup and then the maltitol syrup (containing more than 50% maltitol) is crystallized. The maltitol syrup used as the starting material for crystallization also contains low levels of sorbitol, maltotritol and higher hydrogenated oligosaccharides.
Maltitol in dry form is extremely hygroscopic and deliquescent. As a rule, maltitol has been considered very difficult to prepare in anhydrous crystalline form, e.g. due to its viscosity and solubility characteristics.
A process for preparing anhydrous crystals of maltitol has been described for the first time in U.S. Pat. Nos. 4,408,041 and 4,717,765 (Hayashibara Co.). In this process, an aqueous maltitol solution having a maltitol content of at least 65% (supersaturation degree of about 1.05-1.50) is subjected to crystallization at a temperature in the range of 0-95xc2x0 C. The supersaturation degree and viscosity of the solution can be regulated by adding a water-soluble organic solvent, e.g. methanol, ethanol and acetone. Crystallization of the solution is started at a temperature of about 40-95xc2x0 C. and in a supersaturated concentration, and the content is simultaneously cooled gradually with gentle stirring to obtain a massecuite containing anhydrous crystals of maltitol. The presence of seed crystals in an amount of 0.1-20.00% can accelerate the crystallization. The massecuite thus obtained is then separated into anhydrous crystals of maltitol and mother liquor by conventional separation methods. The method produces anhydrous crystals of maltitol, having a melting point of 146.5-147.0xc2x0 C.
U.S. Pat. No. 4,846,139 (Roquette Frxc3xa8res) discloses a first industrial process for preparing pure maltitol. In this process, the crystallization of maltitol is carried out by cooling crystallization.
U.S. Pat. No. 5,304,388 (Ueno Seiyaku Oyo Kenkyulo Co.) discloses a method of manufacturing powdery or granular crystalline maltitol. In this method, seed crystals of maltitol (e.g. at a temperature of 105xc2x0 C.) are first added to an aqueous solution of maltitol having a moisture content of 1-15% by weight (temperature e.g. 90xc2x0 C.). The mixture is subjected to kneading in the presence or absence of an additive selected from the group consisting of a fat, an oil and a surface-active agent, and a shearing force is continuously applied to the kneaded mass. By applying a shearing force to the kneaded mass, powdery crystalline maltitol can be obtained continuously and at a high yield. In this process, the crystals are not separated from the mother liquor. The product is impure and does not fulfill the requirements of a high quality crystalline product.
U.S. Pat. No. 4,849,023 (Roquette Frxc3xa8res) discloses a method where crystalline maltitol is manufactured from a concentrated maltitol syrup with a 90% dry matter content by maintaining the maltitol syrup four hours at a temperature of 75xc2x0 C., after which spontaneous nucleation starts. The crystallization vessel is then cooled to a temperature of 25xc2x0 C. The crystal mass is drained by centrifugation, and the crystals are dried on a fluidized bed. A crystalline maltitol product is obtained with a yield of 63% and a purity of 99.2% (by HPLC).
In accordance with U.S. Pat. No. 5,651,829 (Roquette Frxc3xa8res), crystalline maltitol can be obtained from a maltitol syrup which has a dry matter content of at least 50% and exhibits a maltitol concentration greater than or equal to 92% by finely atomizing this syrup on a moving pulverulent bed of particles of crystallized maltitol at a concentration which is at least equal to that of the syrup, the bed having a temperature of between 60 and 110xc2x0 C.
European Patent Application EP 0 937 733 A2 (Hayashibara) discloses a continuous method of manufacturing anhydrous crystalline maltitol, where seed crystals are added by mixing to a concentrated solution of maltitol with heating to give a seed crystal-containing massecuite, and the massecuite thus obtained is subjected to disintegration, mixing, stirring and transfer in an atmosphere in which the temperature and moisture are adjusted to cause crystallization to proceed. The last step may be carried out e.g. with hot air having a temperature between 70 and 100xc2x0 C.
In the known crystallization methods described above, the crystallization of maltitol is effected, for example, by cooling, applying shearing forces or drying the massecuite. The typical disadvantages of the cooling-crystallization methods relate to the unfavorable supersaturation gradient between the crystallization mass and heat transfer surfaces. The viscosity and supersaturation are high close to the heat transfer surfaces Heat transfer coefficient is reduced and harmful nucleation is easily generated if cooling is fast. As a result, the crystal mass thus obtained is very difficult to centrifuge. The method is uneconomical, because only a small amount of maltitol is recovered in one step due to the solubility limits. In addition, crystallization rate is low at low temperatures, and crystal size distribution cannot be controlled.
As regards other sugars and sugar alcohols, WO 98/50589 (Xyrofin Oy) discloses a boiling crystallization method of recovering xylose. This reference describes a method of crystallizing xylose where a xylose-containing solution is evaporated to supersaturation at a boiling point of 40 to 80xc2x0 C. of the solution, the solution is seeded, and the evaporation is continued at the boiling point of the crystallization mass, until a crystallization mass with a crystal yield of 1 to 60% and a dry solids content over 70% is obtained.
WO 92/16542 (Xyrofin Oy) discloses a process for the preparation of crystalline anhydrous lactitol by bringing an aqueous lactitol solution to supersaturation in respect of lactitol, and subjecting the solution to crystallization conditions at a temperature above 70xc2x0 C., by evaporating the solution or lowering the temperature under simultaneous stirring, whereupon crystalline anhydrous lactitol is formed. A boiling crystallization method has thus been proposed as one alternative for preparing crystalline lactitol.
Boiling crystallization methods for maltitol have not been suggested in the prior art, obviously because it was not expected that the boiling crystallization could be applied to the crystallization of maltitol, due to the very poor crystallization properties of maltitol. Only crystallization-by-cooling methods have been proposed for the separation of pure maltitol crystals from an impure mother liquid.
The object of the present invention is to provide a method of manufacturing anhydrous crystalline maltitol by a simple procedure and with high efficiency.
In order to achieve the above-mentioned object, there is provided, according to the present invention, a method of manufacturing anhydrous maltitol using a boiling crystallization method. The method of the invention proved feasible for producing high purity crystals from maltitol syrups having varying purity and varying oligosaccharide contents.
The present invention relates to a method of crystallizing maltitol from a maltitol solution by supersaturating the solution in respect of maltitol and crystallizing the maltitol from the solution.
The method of the invention is characterized in that the maltitol-containing solution is brought to a supersaturated state, and the solution is subjected to crystallization by evaporation until a crystallization mass is obtained which has a crystal yield of 1 to 80% on maltitol, and a dry solids content of over 50%.
Preferably, a crystallization mass is obtained which has a crystal yield of 30 to 80%, most preferably 50 to 80% on maltitol, and 65 to 70% on dry solids.
The crystal yields of maltitol above refer to yields immediately after boiling.
The maltitol-containing solution can be obtained e.g. from a starch hydrolysate by methods known per se, including hydrolysis, hydrogenation of maltose to maltitol, and purification e.g. by chromatography or ion-exchange.
The maltitol-containing solution used as the starting material contains preferably at least about 85%, more preferably at least about 88% by weight of maltitol on dissolved dry solids. In the most preferred embodiment of the invention, the maltitol solution contains at least about 92% by weight of maltitol on dissolved dry solids.
The maltitol-containing solution used as the starting material has typically an oligosaccharide (e.g. maltotritol) content less than 4% by weight on dissolved dry solids.
In a preferred embodiment of the invention, the matitol-containing solution is brought to a supersaturated state by evaporation. Preferably, the solution is evaporated to a dry solids content of 80 to 90% by weight. The evaporation is conveniently carried out at the boiling point of the solution at a low pressure, e.g. at 120 to 140 mbar. The evaporation temperature may vary within the range of 50 to 100xc2x0 C., preferably within the range of 55 to 70xc2x0 C.
To form maltitol crystals from a supersaturated solution, seeding is employed. In a preferred embodiment of the invention, seeding is effected by adding seed crystals to the supersaturated solution. As seed crystals, it is possible to use a particulate maltitol powder, typically anhydrous maltitol seed crystals. The amount of seed crystals is preferably 0.001 to 1% by weight of particulate maltitol, based on the maltitol of the crystallization mass, depending e.g. on the size of the seed crystals.
Seeding may also be carried out by using any other known seeding methods, without adding maltitol seed crystals. Seeding may be effected using spontaneous seeding or ultrawave seeding, for example.
Seeding is carried out at a point where a suitable supersaturation has been achieved. As a rule, a suitable seeding supersaturation is within the range of 1.05 to 1.3.
The crystallization by evaporation (after seeding) in accordance with the present invention is conveniently carried out at the boiling point of the solution, under a low pressure. As a rule, the crystallization is carried out at a temperature of 50 to 100xc2x0 C., preferably at a temperature of 55 to 70xc2x0 C. The supersaturation of the solution in respect of maltitol during the crystallization is preferably 1.05 to 1.3. During crystallization by evaporation, the apparent viscosity of the crystallization mass is within the range of 5 to 200 Pa.
During the crystallization by evaporation, the crystal suspension is subjected to boiling and evaporation until a sufficient degree of crystallization (yield, reduction in maltitol purity of the mother liquor, and crystal size) has been achieved. The crystallization by evaporation is usually continued for 1 to 15 hours. During this time, it is possible to achieve a maltitol yield of 1 to 80%, preferably 50 to 80% (crystal yields of maltitol immediately after boiling), and a crystal size of 0.05 to 0.5 mm.
The pH in the crystallization is typically within the range of 4 to 11. During the crystallization by evaporation, the crystal mass is preferably mixed.
In one embodiment of the invention, additional feed liquid is added to the maltitol solution simultaneously with the crystallization by evaporation in order to raise the level of the maltitol solution in the crystallizer and to raise the dry substance content of the maltitol solution. The additional feed liquid can be added continuously or batchwise.
In another embodiment of the invention, the maltitol-containing solution is cooled simultaneously with the crystallization by evaporation (after seeding). In the combined boiling-and-cooling process, the temperature is typically dropped to a temperature which is 10 to 20xc2x0 C. lower than the seeding point temperature. A typical cooling rate is 1xc2x0 C./h to 5xc2x0 C./h.
When the crystallization by evaporation is terminated, the crystal mass is advantageously mixed without cooling. Mixing is preferably carried out under atmospheric pressure at a temperature of 50 to 100xc2x0 C., preferably at a temperature of 55 to 70xc2x0 C. Mixing is conveniently continued for 0.5 to 30 hours.
Alternatively, when the crystallization by evaporation is terminated, the temperature of the crystallization mass is preferably dropped to a range of 70 to 20xc2x0 C. Usually, cooling is effected for 10 to 30 hours. Preferably, cooling is effected to a temperature of 50 to 30xc2x0 C., and especially to 45 to 40xc2x0 C. As a rule, the cooling rate is 1xc2x0 C./h to 5xc2x0 C./h.
If necessary, the supersaturation of the crystallization mass is reduced by raising the temperature and/or diluting the crystallization mass with water or a maltitol-containing solution, so that the viscosity of the crystallization mass drops sufficiently for effective separation of crystallized material. Typically, the viscosity of the crystallization mass is then 50 to 200 Pa.
The crystals can be separated, for example, by centrifugation, filtration, decantation etc., preferably by centrifugation. After the separation step, the crystals are dried, e.g. with hot air. The maltitol content of the crystal fraction obtained is typically over 95%.
In the process of the present invention, the temperature and supersaturation gradient between the heat carrier surface and the crystallization mass is advantageous. The temperature in the vicinity of the heat carrier surfaces is high. Any small crystals may grow, and formation of any new crystal nuclei can be avoided, unlike in the crystallization by cooling. The rate of crystallization is high, since the temperature is suitable and the viscosity of the mother liquor is low, i.e. mass and heat transfer is efficient because of boiling.
The method of the invention makes it very easy to control the crystal size. Also, better output (kg crystals/m3 crystallization mass/h), improved yield and better crystal quality are achieved. Surprisingly, centrifugation of the mass is easy, both by using a batch centrifuge and a continuous centrifuge.
In the specification and the claims, supersaturation of a solution (apparent supersaturation) in respect of maltitol means a dimensionless ratio of the measured maltitol content to the solubility of maltitol, the ratio being calculated from the equation:   s  =            malatitol      ⁢              xe2x80x83            ⁢      content      ⁢              xe2x80x83            ⁢      in      ⁢              xe2x80x83            ⁢      solution              solubility      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      maltitol      ⁢              xe2x80x83            ⁢      at      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      same      ⁢              xe2x80x83            ⁢      temperature      
where s is supersaturation, and the unit of measurement for the maltitol content and maltitol solubility is g/100 g of water. Also, the terms xe2x80x98supersaturatedxe2x80x99 and xe2x80x98supersaturationxe2x80x99 refer to the saturation of the solution in respect of maltitol.
Preferred embodiments of the invention will be described in greater detail by the following examples, which are not to be construed as limiting the scope of the invention.
The following analyses were made of the crystallization samples:
RDS: refractometric dry substance (based on pure maltitol), weight-%.
DS: dry substance with Karl Fischer titration, weight-%.
Colour measured from filtered (0.2 xcexcm membrane) 10% solution; pH 5, 420 nm, 4 cm tube (ICUMSA method).
HPLC (for the determination of carbohydrates): Na+-column; 0.6-0.8 ml/min, T=85xc2x0 C.
DSC (Differential Scanning Calorimeter): 30xc2x0 C.- greater than 200xc2x0 C., 10 xc2x0 C./min.
Moisture: Moisture from the crystals was analysed by coulometric Karl Fischer titration, weight-%.
In the specification and in the attached claims, maltitol purity refers to the proportion of maltitol in the dry solids contained in the solution or mixture. The purity is indicated as % by weight unless stated otherwise.